Cosmology & Gravitation

This series consists of talks in the areas of Cosmology, Gravitation and Particle Physics.

Seminar Series Events/Videos

Currently there are no upcoming talks in this series.

 

Tuesday Nov 03, 2015

Is the graviton a truly massless spin-2 particle, or can the graviton have a small mass? If the mass of the graviton is of order the Hubble scale today, it can potentially help to explain the observed cosmic acceleration. Previous attempts to study massive gravity have been spoiled by the fact that a generic potential for the graviton leads to an instability called the Boulware-Deser ghost. Recently, a special potential has been constructed which avoids this problem while maintaining Lorentz invariance.

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Tuesday Oct 27, 2015

Despite being ubiquitous throughout the Universe, the fundamental physics governing dark matter remains a mystery. While this physics plays little role in the current evolution of large-scale cosmic structures, it did have a major impact in the early epochs of the Universe on the evolution of cosmological density fluctuations on small causal length scales. Studying the astrophysical structures that resulted from the gravitational collapse of fluctuations on these small scales can thus yield important clues about the physics of dark matter.

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Thursday Oct 22, 2015
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Tuesday Oct 20, 2015
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I will talk about a novel theory of dark matter superfluidity that matches the success of LCDM model on cosmological scales while simultaneously reproducing the MOND phenomenology on galactic scales.

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Tuesday Oct 13, 2015
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I will first briefly report the current status on the stability problem of global AdS space under gravitational self-interaction. I will then present evidence that the possibility of a blackhole-forming instability is strongly connected to phase-locked cascade, which is different from the usual energy cascade in turbulent flow.

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Tuesday Sep 29, 2015
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In order for quantum fluctuations during inflation to be converted to classical stochastic perturbations, they must couple to an environment which produces decoherence. Gravity introduces inevitable nonlinearities or mode couplings. We study their contribution to quantum-to-classical behavior during inflation. Working in the Schrodinger picture, we evolve the wavefunctional for scalar fluctuations, accounting for minimal gravitational nonlinearities. The reduced density matrix for a given mode is then found by integrating out shorter-scale modes.

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Tuesday Sep 22, 2015
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I describe an inflation model that can generate a cosmological magnetic seed field of nG strength on Mpc length scales today that could explain observed few microG large-scale galactic magnetic fields. I also summarize some of the extensions of this model that have been developed over the last two decades, as well as open questions about such models.

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Tuesday Sep 15, 2015

The Lagrangian dynamics of a fluid element within a self-gravitational matter field is intrinsically nonlocal due to the presence of the tidal force. Instead of searching for local approximations, we provide a statistical solution that could decouple the evolution of the fluid parcel from the surrounding environment. Given the probability distribution of the matter field, the method produces a set of ordinary differential equations to be solved locally.

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Tuesday Sep 08, 2015
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In this talk I’ll discuss an exotic theory of gravity known as “partially massless” gravity. The linear partially massless theory displays many features analogous to those of electromagnetism, including an electric/magnetic duality. However, the structure of gauge charges is much richer than in E&M. I’ll present the analogues of electric point charges and Dirac monopoles.

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Tuesday Jul 21, 2015

Current and near-future cosmological surveys can provide powerful tests of gravity on the largest scales. However, in order to extract this information, it is essential to understand the host of parameter degeneracies which may arise. I present two approaches towards this goal, with a focus on the use of weak gravitational lensing measurements in combination with other probes. First, I discuss a novel expression for a weak lensing power spectrum under alternative theories of gravity.

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